In a defensive rd for a handgun which is more important to consider....muzzle velocity or muzzle energy?

Of those two factors, energy is more important because energy is the potential to create damage.

As for the rest of the discussion, there are a lot of variables but ultimately it comes down to 2 things. How much energy does a round have and how effectively can it transfer that energy into the target. I shake my head every time someone says bigger and slower is better. If that was true, a 45 would be more effective than a 30-06 and that is obviously not the case. How many here would consider a 5.56 rifle a better weapon than a 45? Probably most of you and rightfully so. How about a 357 magnum vs a 45? Assuming a bullet is placed in the right spot and will transfer it's energy effectively, the round with more energy will do more damage and stop the target more effectively. Now, assuming the energy of two rounds is equal, I would bet on the bigger bullet to transfer it's energy effectively.